A number of viscous flowable pressure compensating fitting materials have been developed in the prior art for a wide variety of utilities. Such materials are generally stable so as to provide firm support and comfort to the user while allowing the pressure compensating fitting material to flow in response to continuous pressure in order to adapt to the particular body part involved. Such compositions and pads based on such compositions are described in U.S. Pat. Nos. 4,144,458; 4,229,546; 4,255,202; and 4,588,229, among others. The compositions described in these patents are, generally speaking, comprised of a mixture of oil, wax, and either glass microbeads or resin microbeads. The microbeads provide the composition with some structure, but because of their relative light weight, the microbeads reduce the density of the composition. Light weight products are desirable and important, particularly in large cushions or pads for wheelchairs wherein the weight of the cushion can be a major factor. The mixture of oil and wax in the composition provides flowability to the composition when subjected to pressure. The oil-wax mixture also maintains a high viscosity in the overall composition, whereby the pad will retain its shape in the absence of pressure.
The prior art products have worked quite well in most applications, but when subjected to temperatures higher than room temperature, such as body temperatures encountered in wheelchair cushion pads, wherein the wheelchair is used as much as 18 or 20 hours per day, 365 days per year, separation of the glass or resin microbeads from the oil-wax mixture occurs to form a hard, non-flowable mass. Separation of the microbeads from the oil-wax results in masses of microbeads which apparently "float" on top of the oil-wax mixture, producing hard, non-flowable lumps which cause discomfort to the user and potential skin breakdown(pressure sones). The tendency of the microbeads to separate is also a problem in vertical applications, such as wheelchair back cushions. Separation of these components has also been observed in cushions which are left in closed automobiles in the sunlight, wherein temperatures above 100.degree. F. are encountered.
Sometimes the separation of the microbeads can be corrected by kneading the pad to redisperse the microbeads in the oil-wax composition. However, when the separation has progressed too far, hard lumps are formed which cannot effectively be redispersed by kneading. Moreover, in many usages, the fitting material is located in inaccessible places which makes it difficult or impossible to knead the flowable pressure compensating material back to its original dispersed form.
The prior art flowable compositions are also extremely temperature sensitive. For example, a pad left in a cold car overnight will be dangerously firm to sit on, as it can't conform to relieve pressure. Conversely, a pad warmed to skin temperature (after 2 hours of sitting) will develop a watery texture and no longer afford the same stability and support as a pad at room temperature. Stability and support are prime requirements in seating for disabled person, as they lose function when sitting on unstable seating surfaces.
It is believed that many of the foregoing problems are related to the wax component of the prior art flowable composition. The wax used in such compositions is temperature sensitive so that it flows and tends to separate from the microbeads at warmer temperatures. The temperature sensitivity also results in a change in viscosity at low temperatures as compared to relatively high temperatures.